Control system for hydrostatic pump

ABSTRACT

A control system that can be used on a hydrostatic pump has a feedback sensor to detect the angle of a swashplate. From the information gathered by the feedback sensor about the swashplate and another set command, this information is sent to a microprocessor. The microprocessor then uses an algorithm to create an output signal and also sends out a superimposed dither signal. This dithered output signal is then received by a pressure control that causes a dither servo pressure in a servo system. Because of the dither pressure the servo system adjusts the position of the swashplate.

BACKGROUND OF THE INVENTION

This invention relates to a control system for a hydrostatic unit. Morespecifically, and without limitation, this invention relates to acontrol system that utilizes a dither signal incorporated with itsnormal input signal in order to adjust the angle of a swashplate.

In the art of hydraulics, oil is pumped by mechanical hydraulic pumpsfor the purpose of causing a hydraulic motor to revolve, a hydrauliccylinder extend, or for other useful purposes. A common aspect of manytractors, earthmoving machines, and the like is a hydrostatictransmission. In its most basic form a hydrostatic transmission consistsof a hydrostatic pump which is normally driven by an internal combustionengine, and provides a source of pressurized oil flow which causes oneor more hydrostatic motors to rotate. The rotation of these one or morehydrostatic motors will cause the machine to travel forward or reverseas commanded by the drive of the machine.

The swashplate is a mechanism in a hydrostatic transmission thatcontrols the fluid flow that a hydraulic pump may deliver. Usually theangle of the swashplate is determined by a hydraulic cylinder or servosystem based on information that a control system or microprocessorreceives. A typical microprocessor uses an algorithm to determine anoutput signal that will adjust the swashplate to a position. The pumpand servo system usually do not match the resolution and accuracy of theinput signal to the hydrostatic unit. This can cause the incorrectpositioning of the swashplate.

Thus, it is a primary object of the present invention to provide acontrol system for a hydrostatic pump that improves upon the state ofthe art.

Another object of the present invention is to use a dither signal toimprove the resolution and accuracy of fluid flow (e.g. swash angleposition) of the pump and/or motor in a hydrostatic control system.

Yet another object of the present invention is to provide a hydrostaticcontrol system that is able to adjust a swashplate angle based on systemparameters using a dithered signal.

These and other objects, features, or advantages of the presentinvention will become apparent from the specification and claims.

BRIEF SUMMARY OF THE INVENTION

The present invention is a control system that controls the angle of aswashplate. The control system can be a microprocessor that receivesinformation from a feedback sensor and setpoint sensor. Themicroprocessor determines not only the angle of the swashplate but alsoa set point command. From the information received from the sensor(s),the microprocessor uses an algorithm to process the information and sendout an output command signal. The microprocessor can simultaneously sendout a superimposed dither signal that produces a resulting signal thatdrives a pressure control. The pressure control in turn causes a servosystem to alter the angle of the swashplate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of the hydrostatic pump of this system;

FIG. 2 is a block representation of the control system of the presentinvention;

FIG. 3 is an enlarged scale portion 3-3 of FIG. 1;

FIG. 4. is an enlarged scale portion 4-4 of FIG. 1; and

FIG. 5 is an enlarged scale portion 5-5 of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The control system of the present invention can be connected to thehydrostatic pump seen in FIG. 1. The hydrostatic pump 10 has a centralhousing 12 of a standard servo hydrostatic pump. A servo piston 14 isdisposed through the central housing 12. The servo piston 14incorporates a servo screw 16, spring guide 18, servo spring 20, andspring seat 22.

The hydrostatic pump 10 also has a swashplate 24. Swashplate 24 isconnected to a shaft feedback assembly 26 that works with the anglesensor assembly 28 to determine the angle of the swashplate 24. Theswashplate 24 connects to the central housing 12 by use of a conebearing 30 that is connected to cup bearing 32 which is then connectedto a first O-ring 34. The first O-ring 34 rests against the bottom ofplate adapter 36. A second O-ring 38 meshes between the plate adapter 36and the bottom of the central housing 12. On the top of the housing is abi-directional pressure control PCP assembly 39 having two coils. As onecan appreciate from FIG. 1 this is a standard servo hydrostatic pump.

FIG. 2 shows in block form a typical closed loop control system can usedto drive hydrostatic pump 10. The control system includes feedbacksensor 40 and an operational parameter setpoint sensor 42. The setpointsensor 42 sends a setpoint signal 44 and the feedback sensor 40 detectsthe angle of the swashplate 24 and sends a feedback signal 46. Both thesetpoint signal 44 and the feedback signal 46 are received by a devicehaving the capability to produce a dithered electrical control signal.This device in a preferred embodiment is a microprocessor 48 aspictured; however, in alternative embodiments the device may be anelectric joystick, an electric foot pedal control, or any other devicethat can produce a dithered output signal. The microprocessor 48 thensends out a control signal 50 comprised of an average signal and adither signal. A pressure control 52 then receives the control signal 50and creates a pressure signal 54 or 56 that is sent to a servo system58. The servo system 58 then produces a force 60 that alters theposition of swashplate 24. It should be appreciated that the feedbacksensor 40 is optional as the system can improve performance in a closedloop as well as open loop mode.

In the microprocessor 48 an error signal is generated which is processedthrough a typical closed loop algorithm. These algorithms include, butare not limited to, PID, PID plus feed forward, and KIDT1. From theoutput of the algorithm(s) the microprocessor 48 creates an outputsignal that is superimposed with a dither signal generating theresulting signal 50 that is received by pressure control 52. It shouldbe appreciated that the pressure control 52 can be of any type,including but not limited to a flapper nozzle style pilot valve with twoboost spools, a flapper nozzle style pilot valve with one boost spool, aflow control (a device that converts an electrical signal into anhydraulic signal to position the swashplate), or a plurality of pressurecontrols.

The pressure control 52 responds to the output signal and the dither togenerate a dithering servo pressure. Based on this dithering servopressure the max slew rate of the swashplate 24 is determined. The pumpswashplate 24 position is therefore determined by the typical forcebalance of the servo springs, servo pressure, pressure moments, speedmoments, and other system factors.

It should be appreciated that the dither signal created by themicroprocessor 48 can be independent or dependent of the swash anglevalue related by feedback sensor 40. Furthermore, the feedback sensor 40and the closed loop algorithm used by the microprocessor can be anystandard feedback sensor or algorithm. In the current embodiment themicroprocessor 48 generates the dither signal; however, the dithersignal could also be generated internally in the pressure control 52 orexternally in another device.

It will be appreciated by those skilled in the art that other variousmodifications could be made to the device without the parting from thespirit in scope of this invention. All such modifications and changesfall within the scope of the claims and are intended to be coveredthereby.

1. A control system for a hydrostatic unit having a swashplatecomprising: an electronic means for producing a dithered output signal;a pressure control adapted to receive the dithered output signal andposition the swashplate.
 2. The control system for a hydrostatic unit ofclaim 1 wherein the electronic means is a microprocessor.
 3. The controlsystem of claim 2 wherein the microprocessor receives information from aset point command signal.
 4. The control system of claim 2 wherein themicroprocessor receives information from a feedback sensor.
 5. Thecontrol system of claim 1 wherein the pressure control is a flappernozzle style pilot valve with two boost spools.
 6. The control system ofclaim 1 further comprising: a servo system operably connected to thepressure control and swashplate.
 7. A method of controlling the angle ofa swashplate of a hydrostatic unit having a swashplate comprising stepsof: generating an electric signal based on a set point signal; receivingthe electric signal in a microprocessor; interpolating the informationfrom the electric signal using an algorithm contained in themicroprocessor; sending an output signal from the microprocessor to apressure control; dithering the output signal; and generating a ditheredpressure from the pressure control that displaces the swashplate.
 8. Themethod of claim 7 wherein the set point signal is generated by measuringan operational parameter.
 9. The method of claim 8 wherein theoperational parameter is the angle of the swashplate.
 10. The method ofclaim 7 wherein the algorithm is a PID type algorithm.
 11. The method ofclaim 7 wherein the algorithm is a PID+feed forward algorithm.
 12. Themethod of claim 7 wherein the algorithm is a KIDT1 algorithm.
 13. Themethod of claim 7 wherein the pressure control is a flapper nozzle stylepilot valve with two boost spools.
 14. The method of claim 7 wherein thepressure control is a flapper nozzle style pilot valve with one boostspool.
 15. The method of claim 7 wherein the pressure control is a flowcontrol.
 16. The method of claim 7 wherein the pressure control iscomprised of two pressure controls.
 17. The method of claim 7 whereinthe output signal is dithered by the pressure control.
 18. The method ofclaim 7 wherein the output signal is dithered by the microprocessor. 19.A control system for a hydrostatic pump having a swashplate comprising:a feedback sensor adapted to sense the angle of the swashplate; amicroprocessor adapted to receive information from the feedback sensorand produce a dithered output signal; a pressure control adapted toreceive the dithered output signal and position the swashplate.
 20. Thecontrol system of claim 19 wherein the microprocessor is also adapted toreceive information from a set point command signal.